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ment of parts, and do not at all affect the general plan or arrangement of the 

 organs appropriated to those functions. 



To the Zoologist the single systemic heart of Nautilus affords an interesting 

 example of its affinity to the inferior Mollusks ; whilst at the same time it tends 

 materially to illustrate the relations which subsist between the locomotive, cir- 

 culating, and respiratory functions. In comparing the branchiae only of the 

 two orders of Cephalopoda, the increased number of these organs in the type of 

 the Tetrabranchiata, appears at first sight to compensate for their diminished 

 size ; but when we reflect on the perfection, as a respiratory apparatus, which 

 the gills of the Dibranchiata derive from the ventricles appropriated to acce- 

 lerate the circulation through them, we are then led to consider it as one of the 

 chief causes of their superiority over all other Mollusks in locomotive energies, 

 manifested in the rapidity with which, by means of varied fin-like structures, 

 they propel themselves through the water. 



Whilst the organization of the Cephalopods was known only in the Dibran- 

 chiata, the presence of a branchial ventricle might naturally be supposed to 

 depend on the pecuUar structure of the gill: thus it has been observed, " In 

 the Teredines the water is intimately applied to the giUs from the simplicity of 

 their structure ; but in the Sepia they are more complex, and require force to 

 apply the water to every part of them ; and for this purpose there is a bulb and 

 double valve placed at the root of each gill." — Home, Lectures on Camp. Anat. iv. 

 p. 164. But the same structure of gill occurring in Nautilus without any ven- 

 tricles being appended to the breathing organs, notwithstanding the added com- 

 plexity of subdivision, renders necessary a consideration of the subject in its 

 other relations, and irresistibly leads to the above conclusion that the Mollusks, 

 which execute rapid and vigorous movements in water, require that degree of 

 perfection in their respiratory system which is afforded by a superadded muscular 

 power at the commencement of the lesser circulation. 



The interesting character which Nautilus sustains as an osculant form be- 

 tween Cephalopoda and Gasteropoda, will perhaps be considered to have been 

 sufficiently manifested by the instances of affinities already cited ; but it is ren- 

 dered still more evident on a consideration of the nervous system. The forms, 

 proportions, and disposition of the principal masses of this system, appeared, 

 indeed, at first sight to recede so far from the type of the higher Cephalopods, 



